Volcanoes

8th Grade Earth & Space Science - Class Notes

Zones of Volcanism

• Volcanism – describes all processes

associated with the discharge of magma,

hot fluids, and gases

• Most volcanoes form at plate boundaries.

• The majority form at convergent

boundaries and divergent boundaries.

Zones of Volcanism

Convergent Volcanism

• Remember In an oceanic-continental

subduction zone, the denser oceanic plate

slides under the continental plate into the

hot mantle.

• Parts of the plate melt and magma rises,

eventually leading to the formation of a

volcano.

Convergent Volcanism

• Most volcanoes located on land result

from oceanic-continental subduction.

• These volcanoes are characterized by

explosive eruptions.

Two Major Belts

• The volcanoes associated with convergent

plate boundaries form two major belts.

• The larger belt, the Circum-Pacific Belt,

is also called the Pacific Ring of Fire. The

outline of the belt corresponds to the

outline of the Pacific Plate.

Two Major Belts

Two Major Belts

• The smaller belt is the Mediterranean

Belt.

• Its general outline corresponds to the

boundaries between the Eurasian, African,

and Arabian plates.

Two Major Belts

Divergent Volcanism • Eruptions at divergent boundaries tend to

be nonexplosive.

• Many occur along the Mid-Atlantic ridge.

• At the divergent boundary on the ocean

floor, eruptions often form huge piles of

lava called pillow lava.

Hot Spots • Some volcanoes form far from plate

boundaries over hot spots.

• A hot spot is an unusually hot area in

Earth’s mantle where high-temperature

plumes of mantle material rise toward the

surface.

Hot Spots and Hawaii • The Hawaiian islands are located over a

plume of magma.

• The hot spot formed by the magma plume

remained stationary while the Pacific Plate

slowly moved northwest.

Hot Spots and Hawaii • The volcanoes on the oldest Hawaiian

island, Kauai, are inactive because the island no longer sits above the stationary hot spot.

• The world’s most active volcano, Kilauea, on the Big Island of Hawaii, is currently located over the hot spot.

Lava flow at Kilauea

Hot Spots and Plate Movement • The rate and direction of plate motion can

be calculated from the positions of hot

spot volcanoes.

Hot Spots and Plate Movement • The Hawaiian islands are at one end of

the Hawaiian-Emperor volcanic chain. The

oldest seamount, Meiji, is at the other end

of the chain and is about 80 million years

old.

Hot Spots and Flood Basalts

• Flood basalts form when lava flows out of

long cracks in Earth’s crust.

• These cracks are called fissures.

Columbia River Basalts

Deccan Traps

• About 65 M.Y.A. in India, a huge flood

basalt eruption created an enormous

plateau called the Deccan Traps. The

volume of basalt in the Deccan Traps is

estimated to be about 512,000 km3.

Parts of a Volcano • Conduit – tube like structure that lava

travels through to reach the surface

• Vent - opening that lava emerges through

• Over time, layers of solidified lava can

accumulate to form a mountain known as

a volcano.

Parts of a Volcano

• Crater – bowl-shaped depression found at

the top of the volcano surrounding the vent

Parts of a Volcano • Caldera – large volcanic crater; often

forms after the magma chamber beneath a

volcano empties after a major eruption

– the summit or side of the volcano collapses

into the empty chamber and leaves a large,

circular impression

Crater Lake

Formed when Mount Mazama collapsed in approximately 5,677 B.C.

Types of Volcanoes

• The appearance of a volcano depends on

two factors:

– the type of material that forms the volcano

– the type of eruptions that occur

Shield Volcanoes

• Largest

• Long, gentle slopes

• Composed of layers of solidified basalt

lava

• Quiet explosions

Mauna Loa in Hawaii

Cinder Cones

• Smallest, but steep

• Forms from small pieces of magma (tephra) that

falls back to Earth and piles up around the vent

• Cone-shaped

• Usually basaltic lava

• Explosive eruptions

Cinder Cone at Lassen Park

in California

Composite Volcanoes • Composed of layers of hardened chunks of lava

from violent eruptions alternating with layers of

lava that oozed downslope

• Cone-shaped but larger than cinder cones

• Also called “stratovolcanoes”

• Tend to have explosive eruptions with cycles of

quiet

Mt. Rainer near Seattle

Identify the types of

volcanoes.

Section 18.2 - Eruptions

8th Grade Earth & Space Science - Class Notes

Making Magma

• The type of eruption depends on the composition of the magma.

• Remember the formation of magma is effected by: – Temperature

– Pressure

– Water content

– Chemical composition

Composition of Magma

• Explosivity – how a volcano erupts and how its magma flows

• Factors -

– Interaction with overlying crust

– Temperature

– Pressure

– Dissolved gases

– Silica content

Dissolved Gases

• As the amount of gases increases, the

magma’s explosivity increases

• Important gases:

– Carbon dioxide

– Water vapor (most common)

– Sulfur dioxide

– Hydrogen sulfide

Viscosity • Viscosity – physical property that

describes a material’s resistance to flow

• Cooler magma = higher viscosity

• High silica = higher viscosity

• Higher viscosity tends to trap

gases and produces explosive

eruptions

Types of Magma

• The silica content of magma determines

not only its explosivity and viscosity, but

also which type of volcanic rock it forms as

it cools

• Three types:

– Basaltic

– Andesitic

– Rhyolitic

Basaltic Magma • Usually forms from rock in the upper

mantle

• Less than 50% silica – low viscosity

• Gases escape easily

• Quiet eruptions

Examples – Mauna Loa, Kilauea, Surtsey

Basaltic lava flow from Kilauea in Hawaii

Andesitic Magma • 50-60% silica

• Found along oceanic-continental subduction

zones

• Forms from oceanic crust or oceanic

sediments

• Intermediate viscosity

• Intermediate explosivity

Examples – Colima, Tambora

Andesitic magma flow

from Colima in Mexico

Rhyolitic Magma • Molten material that rises and is mixed with

continental crust (rich in water and silica)

• More than 60% silica

• High viscosity

• Large amount of trapped gases

• Very explosive

Example – Chaiten in

Chile

Photo taken of the 2008 eruption

of Chaiten in Chile

Explosive Eruptions

• When lava is too viscous to flow freely

from the vent, pressure builds up in the

lava until the volcano explodes

– There are two major effects from these types

of eruptions – tephra and pyroclastic flows

• Tephra – erupted materials given off by

the volcano; can be pieces of solidified

lava or pieces of crust

Tephra • Tephra – erupted materials given off by the

volcano; can be pieces of solidified lava or pieces of crust

• Classified by size

• Smallest – ash

– Can rise very far in the air

– Threatens aircrafts

– Can affect weather

• Largest – blocks

– Can be as large as a car

Pyroclastic Flows • Pyroclastic flow – rapidly moving clouds

of tephra mixed with hot, suffocating gases

• Can reach temperatures of 1000°C

• Can move at more than 700 km/h

Pyroclastic flow

rushes down side of

Mayon Volcano,

Philippines